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Most cars I've worked on in the past (GM products) gave a direct torque setting for bolts.

It was only when I started working on my M5 that I ran into 'torque and angle'

What's the advantage ?
 

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on your torque wrench you have 1 to 5% tolerance
60 degrees is always 60 degrees
a low torque with a number of degrees has less deviation
 

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Most cars I've worked on in the past (GM products) gave a direct torque setting for bolts.

It was only when I started working on my M5 that I ran into 'torque and angle'

What's the advantage ?
First, What is a head bolt is doing?
The head must be held to the block with enough tension to prevent the combustion chamber pressures from escaping yet the tension must not excessively crush the head gasket. Also the tension must remain somewhat constant even with an aluminum head when the engine is cold up to the maximum anticipated operating temperature of the head. Of course, all bets are off if the operating temperature range is exceeded.

The way a threaded fastener holds two parts together is the same principal as a high rate spring.
The fasteners shank diameter, shank length and hardness determines the optimum tension and elastic range for a specific fastener.
Hence, a head bolt is designed long enough so it can provide a constant tension with the head expansion and contraction.
As a design consideration, a head bolt with a 4" shank can provide a 2X greater range of expansion/contraction while maintaining a desired tension than one with a 2" shank.

Also, if a fastener is not tightened enough it is not placed under sufficient tensioned and that will allow the tension between the parts to very greatly under the dynamic thermal conditions of operation.
If a fastener is tightened too much the fasteners yield point is exceeded (plastic/permanent deformation) and the fastener becomes weaker long before it fail/breaks.

Torqueing a fastener by lb-ft or Nm is a simple method to predictably set the fastener tension without exceeding the fasteners yield point.
The problem is tensioning errors are caused by variations in fastener head bearing and thread friction. These normal frictions must be considered by the designer when specifying lb-ft or Nm torque values for a fastener.

The process of setting a relative low initial torque value and then rotating the fastener by degrees is a more accurate method of setting the fastener's tension. The angle degree method uses the known pitch of the threads mm/turn metric (threads/inch SAE) to set the specific fasteners tension eliminating the tensioning errors caused by variations in fastener head bearing and thread friction.

Also, never, ever use a torque/angle spec with an unknown fastener because the torque and angle values are specific to the fastener's specification and not the application. Also, always replace the head bolts because the tension value is only as good as the individual fastener and removing and re-tensioning head bolts is not a good practice. Also, also the threads in the block must be super clean and free of oil, liquids and foreign debris to prevent cracking threads in the block when the head bolts are installed.
 

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Most cars I've worked on in the past (GM products) gave a direct torque setting for bolts.

It was only when I started working on my M5 that I ran into 'torque and angle'

What's the advantage ?
Really. Those were some ancient GM products. Most GM Powertrains have been using a torque+angle strategy from the mid-'80s and all by the late-'90s.
 
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